Anything by Kernighan/Pike/Ritchie such as The C Programming Language, The Unix Programming Environment and gopl.io book are all concise and well written books with just exactly the right amount of material it seems. Paolo Aluffi's Algebra Chapter 0 is a good math text as a math example for something that is more than just dumping propositions/theorems and proofs.

I liked this draft book as an algorithms book example http://www.parallel-algorithms-book.com/

MIT's course notes are good as brief material that compliments the lectures https://ocw.mit.edu/courses/electrical-engineering-and-compu...

Knuth also labels the difficulties of each problem set in his books like Concrete Math, something I wish more authors did. Concrete Math is also a good example of a well written and engaging math book accessible to a motivated highschool student.

I had 3 beginning stats books and they were all terrible. I used "statistics for the utterly confused" and it helped click everything together with about 1000 less pages.